Conventionally, a cutting method of a glass plate consists mainly of scribing the glass plate by means of an ultra-hard material, such as diamond, to generate a scribe line of the glass plate, which is then subjected to a breaking process under mechanical stress.
Since Lumley has firstly reported a method of cutting a glass plate with a laser beam in Ceramic Bulletin, Vol. 48, No. 9, 1969, much research therefor has been performed and filed.
According to the studies of Lumley, a glass material is not cut by heating, but cut by generating shallow cracks (hereinafter, referred to as ‘scribe line’) overheated and expanded by a laser beam and propagating them. Then, U.S. Pat. No. 3,932,726 discloses a method of severing a glass plate having an unlimited length to plates having a predetermined length. In U.S. Pat. No. 6,112,967, there is disclosed a method of generating a shallow scribe line by irradiating a laser beam in a “U” shape on a target material and then cooling the target material. Further, U.S. Pat. No. 5,609,284 discloses a method of generating a deep scribe line by pre-heating a glass plate.
Before the laser beam irradiation for generation of the scribe line, a micro-crack is mechanically formed in a glass plate and then propagated (U.S. Pat. No. 6,252,197), or a crack is formed in a substrate by use of a pulse laser beam (U.S. Pat. No. 6,211,488).
Recently, there are proposed methods of cutting a single-crystal silicon wafer, as a semiconductor material, by use of a laser.
However, the above laser beam cutting method, characterized in that the scribing process to generate the scribe line uses the laser beam and the breaking process employs a mechanical stress, is disadvantageous in terms of low reliability, and additional requirement of a polishing process due to the breaking process by the mechanical stress.
To solve the above problems, a laser cutting method of a non-metallic material, such as a glass plate, is disclosed (Korean Patent Application No. 10-2000-0042313), which includes forming initial cracks in a desired direction at a cutting initiation point of the non-metallic material, irradiating a first heating beam along the cutting line to heat the non-metallic material, firstly quenching the heated portion by the first beam to generate cracks, irradiating a second heating beam to the cracks to heat the non-metallic material, and secondly quenching the heated portion by the second heating beam.
That is, the above method is characterized in that not only the generation of the initial cracks and the scribing process, but also the breaking process, are performed by use of the laser beam. Thereby, a cutting efficiency of the glass plate can increase up to 95% or more.
However, cracks having irregular sizes and shapes, referred to as hackle marks at a section of the cutting initiation portion of the glass plate, constitutes 10% of the entire cut portions, and thus the glass plate has drastically uneven surfaces on the section thereof, which negatively affects the quality of the end product.
Although the hackle marks are shown upon the initial cutting of the glass plate, the cut section becomes smooth after the initial cutting. Thereby, a rectilinear-cutting characteristic of the glass plate decreases at the cutting initiation portion thereof, and thus complete separation of the plate is not achieved.
Further, evenness of the cut section of the glass plate reduces, resulting in decreasing both outer appearance and quality of end product. Also, while the plate is separated, small fragments may be generated.
In cases where the glass plate is cut at an equal rate to solve the above problems, the sizes and numbers of the hackle marks formed on the section of the plate may decrease according to output conditions of the laser, however, the hackle marks cannot be completely removed.
Korean Patent Application No. 10-2002-65542, filed by the present inventors to overcome the above problems, suffers from insufficient cutting conditions of the glass plate.